C. Q. was recently exposed to group A hemolytic Streptococcus and subsequently developed a pharyngeal infection. His clinic examination reveals an oral temperature of 102.3°F, skin rash, dysphagia, and reddened throat mucosa with multiple pustules. He complains of sore throat, malaise, and joint stiffness. A throat culture is positive for Streptococcus, and antibiotics have been prescribed. The etiology of C. Q.’s disease is